DB
Overview
The term "dB" is used to describe a ratio between two audio levels. As such; it has no absolute value. Due to the non-linear nature of human hearing, the logarithmic dB scale approximates the relationship of the measured value to the perceived change in acoustic level.
Basics
Please note: due to limitations in supported text characters; in the following discussion a value such as "two squared" is described as "2 raised to the power of 2" for clarity.
The decibel may be defined in this manner: two amounts of power differ by 1 decibel when they are in the ratio of 100 raised to the power of one-tenth. The term was used originally in early telephony to measure loss in a standard mile of telephone wire. In honor of Alexander Graham Bell, and to indicate the "decimal" power relationship; the unit was named the "decibel."
The ratio can be expressed as 10 raised to the power of (dB x 0.1); so a 6dB difference between two signals means the larger signal equals the value of the smaller signal multiplied times (10 raised to the power of 0.6) or "ten to the six-tenths power."
In order to give a dB measurement an absolute value; it must have a “zero reference.” One analogy is temperature in “degrees.” Without knowing what “zero degrees” is; we can only talk about the difference between two temperatures (a form of ratio). Unfortunately; the analogy fails when you bring in the differences between Fahrenheit and Centigrade because (unlike the decibel) the “degree” has different definitions in each system!
One of the more common forms of “absolute” dB scales is Sound Pressure Level or “SPL.” In this case, the “zero reference” is a very small value and the scale only extends in the positive value direction. The reference acoustic level is considered to be the “threshold of human perception” and SPL is thus a scale that tells us how much louder a sound level is than the quietest sound one can perceive.
dBu
In audio electronics, the “0dBm” standard for power was 1 milliwatt. Because most early audio transmission was based on 600 Ohm systems, this power level was achieved when an RMS voltage of 0.775 volts was applied to a 600 Ohm load. To make this system applicable to pure voltage level measurements; the “dBu” scale is used with the same zero reference of 0.775 Volts.
When the use of VU meters (“Volume Unit”) became prevalent; the VU meter’s “0” was defined as “+4dBm” and in contemporary systems is defined as “+4dBu.”
dBV
The dBV scale is typically used in consumer audio measurements and ratings. The zero reference for dBV is 1 Volt rms.
“+4” versus “-10”
Professional line level is often referred to as “+4” as versus the consumer line level of “-10”; which is source of confusion. This is because “+4” is “+4dBu” and “-10” is “-10dBV” and the dBV scale uses a different “zero!” The result is that, rather than the “apparent” difference of 14dB between “+4” and “-10”; the actual difference is closer to 12 dB (11.8dB).
dBFS
The term dBFS or “dB Full Scale” is used to describe the level of a linear PCM digital audio signal and has no fixed relationship to dBu or "analog level." Depending on the calibration of the AD converter used to encode the audio signal; a wide range of analog levels can result in the same digital level. Because no signal information is retained once the signal level exceeds 0dBFS; the most important level for calibration purposes is peak level. Professional level analog audio equipment typically has a peak output level of between +18 and +24dBu. Because the standard for VU meters is “0dBVU = +4dBu,” this results in “0dBVU = -14dBFS” for a peak level of +18dBu and “0dBVU = -20dBFS” for +24dBu. The use of “-14” as a calibration standard for digital audio gear thus means the analog input is calibrated to accept a peak analog level of +18dBu.